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Curator of Communications John Liffen blogs on recreating early ‘wireless telegraphy’ ahead of the opening of Information Age, a new gallery exploring communication technologies.

Radio operators on board ship used to be nicknamed ‘Sparks’ – and with good reason. In the earliest days of ‘wireless telegraphy’ the radio waves were created by a continuous train of high-voltage sparks. The phenomenon can be observed whenever you switch a light on or off if a radio set is switched on nearby. At the moment the switch connects or breaks the mains circuit, sparks are created which send out pulses of radio waves which will be heard as a momentary crackle through the set’s loudspeaker.

On board ship, generating equipment continuously charged up capacitors which spontaneously discharged at high voltage across a spark gap. The radio operator used a morse key (a simple on/off switch) to interrupt the sparks in the form of morse code. The resulting coded radio wave ‘oscillations’ could be heard by other ships within a radius of a hundred miles or so. This was the method of transmission in use at the time of the Titanic disaster in April 1912. Jack Phillips and Harold Bride, the operators on the Titanic, sent out their distress signals in this way which were picked up through the headphones of operators on board other ships with range.

The Titanic, seen shortly before she sailed on her last voyage (Source: Science Museum / SSPL)

In the 1920s the use of spark transmission was phased out as more efficient transmitters using thermionic valves or ‘tubes’ were introduced. These could be tuned to a much more precise frequency so were less wasteful of the radio-frequency spectrum than spark transmitters. Today almost nobody will have heard the sound of a radio message sent by spark.

Consequently when we decided to feature the Titanic disaster in our new Information Agegallery, we felt we should re-create some of the morse distress messages so they could be heard by visitors much as they had been in 1912.

In order to do so we had to find a specialist radio historian with a suitable spark transmitter. One such individual is Dr Tony Constable, the founding Chairman of the British Vintage Wireless Society. Recently I visited his home in west London.

He had set up a suitable induction coil, Hertz-type spark gap and morse key in his living room, set to work at the lowest possible power so as to avoid, as far as possible, interference on neighbours’ radios.

Tony Constable keys in a message. The spark transmitter is on the right (Source: John Liffen / Science Museum)

Together we worked out a couple of messages, adapted from the original transcripts. This from the Titanic:

SOS SOS SOS CQD CQD CQD TITANIC STRUCK ICEBERG

and this from the rescue ship Carpathia:

TITANIC CALLS CQD HIS SIGNALS BLURRED AND END ABRUPTLY

CQD was the original morse distress signal. It had been replaced by SOS before 1912 but at that time both were still used by some operators.

Tony placed an vintage Bush transistor radio on the other side of the same room and de-tuned it on medium wave away from any programmes so that it just gave out a hiss. The morse signals were clearly audible as a harsh buzzing, incidentally demonstrating the un-tuned nature of spark transmission.

How the signals were received: Bush transistor radio on left, digital recorder on right (source: John Liffen / Science Museum)

As you can hear, the recordings were very successful. A word of warning, though. The equipment, though simple, uses very high voltages and must be handled with extreme care. We don’t recommend you try this particular experiment yourself.

Discover more about the history of communication technologies in our new Information Age gallery, opening in 2014.

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